Add missing CREATE TABLE IF NOT EXISTS table_name AS EXECUTE query;

Hi,

The first example below works while the second one is a syntax error 
despite that the documentation 
(https://www.postgresql.org/docs/11/sql-createtableas.html) makes it 
seem like both should be valid. I do not see any reason to not support 
CTAS with both IF NOT EXISTS and EXECUTE at the same time so i am 
guessing that this was an oversight.

I have attached a patch which fixes this. What do you think? Should I 
add a new test case for this or is the change simple enough to not 
require any new test?

# CREATE TABLE IF NOT EXISTS a AS SELECT 1;
SELECT 1

# PREPARE q AS SELECT 1;
PREPARE
Time: 0.209 ms
foo=# CREATE TABLE IF NOT EXISTS a AS EXECUTE q;
ERROR:  syntax error at or near "EXECUTE"
LINE 1: CREATE TABLE IF NOT EXISTS a AS EXECUTE q;
                                         ^

Andreas